Biaxially stretched polypropylene films have excellent electrical properties, such as voltage resistance and low dielectric loss characteristics, and high moisture resistance. By utilizing these properties, biaxially stretched polypropylene films have been suitably used as dielectric films for capacitors, such as high voltage capacitors, filter capacitors for various switching regulators, converters, inverters, and the like, and smoothing capacitors, among electric and electronic devices. Furthermore, polypropylene films have started to be used also as capacitors for inverter power supplies that regulate drive motors of electric cars, hybrid cars, and the like which have been increasingly in demand in recent years.
For such capacitors for inverter power supplies used in automobiles or the like, further reductions in size and weight of capacitors have been demanded along with the reductions in size and weight of vehicles. To reduce size and weight of a capacitor, as a film for capacitors, a polypropylene film with high stretching performance may be used as a capacitor film and formed into an ultrathin film, for example. However, since an ultrathin film readily causes dielectric breakdown when a high voltage is applied, polypropylene films having even higher voltage resistance have been required.
To obtain a polypropylene film having such high voltage resistance, molecular designing of the polypropylene or the like have been performed. Examples thereof include a method in which the polypropylene resin components are selected in a manner that the molecular weight distribution is widely distributed in low molecular weight side, and the like. By allowing the molecular weight distribution to be widely distributed in the low molecular weight side, appropriate resin flowability is exhibited during biaxial stretching, and both the voltage resistance and thin film thickness can be achieved. Examples of the method to adjust such a molecular weight distribution include a method in which polypropylene having a high melt flow rate, that is, polypropylene having a low molecular weight, is mixed to a polypropylene resin, and the like (e.g. Patent Document 1). Examples thereof further include a method in which the proportion of polypropylene having a low molecular weight in the polypropylene resin is increased by selectively subjecting polypropylene having a high molecular weight to peroxidative decomposition treatment using peroxide (e.g. Patent Document 2), and the like.
Furthermore, examples thereof further include a method in which size of spherulites, produced in a step of cooling a melt-extruded resin sheet by adding branched chain polypropylene which is a high melt tension polymer for a polypropylene resin, is controlled to be small (e.g. Patent Document 3). Furthermore, Patent Document 3 further discloses a method to control a surface of a film by adding a small amount of branched chain polypropylene.
As described above, various methods have been employed to obtain polypropylene films for use in capacitors. However, to further reduce size and weight of a capacitor, further voltage resistance of a polypropylene film that has been formed into a thin film is required.